Citigroup Inc., one of the world’s largest credit-card issuers, said it will refund $335 million to U.S. customers whose annual percentage rate should have been lower. The lender determined that a method it was using to calculate APRs didn’t properly reflect the full benefit customers should have received for good behavior, such as paying on time, the New York-based bank said Friday in a securities filing that disclosed the issue and the total cost.